Baking ovens with cooling fans, on the one hand, protect sensitive components, especially electronic controllers and parts in the vicinity, from overheating and, on the other hand, remove excess steam from the cooking chamber. They also prevent excessive concentrations of steam in the cooking chamber and also prevent outflow of steam at sites of leakage. Since different types of baked goods generate different volumes of steam at comparable temperatures, and because steam condensates on cooler surfaces in a manner which strongly depends on the present condition, especially of the baking oven wall, previously used methods of controlling the cooling fan based on the heating power of the oven, or the inside temperature thereof, are not satisfactory.
German Patent Application DE 38 04 678 A1, for example, describes a method for controlling the exhaust flow from a cooking chamber. The exhaust fan is controlled as a function of the humidity measured in the vapor exhaust duct during the cooking process.
German Patent Application DE 102 11 522 A1 describes that the speed of a fan for generating an air flow in the cooking chamber can be adjusted between zero and a maximum speed to thereby control the extraction of air from the cooking chamber. It is proposed to use an oxygen sensor for purposes of controlling the exhaust air volume. 
Furthermore, German Patent DE 29 25 947 C2 describes the general relationship that a measured value is obtained in a first time interval, and the controlling variable determined therefrom is used in a subsequent second time interval.
In order to control the exhaust flow rate by fan speed control in a manner that is adapted to the cooking process, it is proposed in EP 1 156 282 to measure, as a control parameter, at least one physical parameter which varies with the pressure difference between the interior of the oven and its environment. This ensures that the fan speed is controlled in a manner that is adapted to the specific cooking process. To this end, the temperature is measured during a period of time, a predetermined setpoint being provided which, when exceeded by the temperature, causes the fan speed to be adjusted upward to the point where the temperature falls below a predetermined setpoint. When another temperature change occurs above the upper setpoint, the control of the fan is increased again, and so on.
In this fan speed control method the fan speed is dependent on the heating temperature in the oven, the temperatures being adjusted downward by the discharge or exhaust flow in a suitable manner, so that the resulting temperature variation during the cooking process occurs between a lower and an upper setpoint.
This type of fan speed control is believed to have the disadvantage that speed control is performed continuously, which, in particular, ties up computing power in the controller.
In order for an appliance to operate in an optimal manner, the extraction of cooking vapors from the cooking chamber must be performed in such a way that no cooking vapors exit the cooking chamber at unwanted sites, air intake openings, leaks, due to positive pressure. The cooking vapors are intended to be discharged from the baking oven by flow-rate controllable extraction only through the vent that is provided for this purpose and, if present, through the oxidation catalyst located therein. To achieve this, a minimum extraction rate is required. A sensor system, as described, provides information on the extraction rate required for this purpose. The lower the extraction rate, the lower are the energy losses of the cooking appliance. In prior art methods, the required extraction power is generally not adequately adjusted to the demand. Realization that the fan speed is correlated with the baking oven temperature is described in EP 1 156 282. In this context, it is assumed that the amount of cooking vapors produced in the oven chamber at high temperatures is greater than at low  temperatures, and that, therefore, a higher extraction rate is required at high temperatures. However, there is no close correlation with the actual demand.